Solar arrays are widely used to provide power for both space and terrestrial applications. In space, for example, solar arrays provide power for spacecraft and the like. With respect to terrestrial applications, solar arrays oftentimes replenish storage batteries and provide power for equipment and instrumentation located remote from conventional power sources.
Solar arrays are generally comprised of a number of solar array strings. The solar array strings are combined, such as in a parallel arrangement, to provide power via an output power bus. The power level of the output power bus is desirably maintained at a predetermined level that may remain constant or that may vary in a preset manner over time. Unfortunately, the power provided by a conventional solar array varies based upon a number of factors such that its unregulated output generally will not remain at the desired power level. The factors that can alter the output of a solar array include temperature variations of the solar array and its surroundings and variations in the solar radiation incident upon the solar array. Among other things, the variations in the solar radiation that is incident upon the solar array include variations in the angle of incidence and the magnitude of the intensity of the radiation incident upon the solar array.
Therefore, regulators have been developed to control the output power of a solar array. These regulators are typically classified as either a series-type or a shunt-type regulator depending upon whether the active element of the regulator is in series with the solar array or in parallel with the solar array, respectively. Conventionally, series-type regulators have difficulty providing acceptable switching characteristics. These switching characteristics include, for example, the power requirements of each switch that connects a respective solar array string to the output power bus as well as the large number of switches required to control the solar array strings and the associated control logic required to control the switches. As such, a variety of shunt-type regulators have been developed.
As will be apparent, shunt-type regulators control the delivery of power to the output power bus by selectively connecting a respective solar array string to either the output power bus or to ground or some other load in order to dissipate the energy provided by the solar array string. While shunt-type regulators are acceptable for certain applications, shunt-type regulators are generally constructed of numerous analog components, including a substantial number of switches and associated control circuitry. Because of the relative large number of components, shunt-type regulators generally require substantial fabrication time and are correspondingly expensive. In addition, shunt-type regulators are oftentimes relatively large and heavy, thereby limiting the number of applications for which shunt-type regulators are acceptable. Since shunt-type regulators must generally include dissipative elements for dissipating the energy generated by the respective solar array strings, a shunt-type regulator is also susceptible to thermal stress failures due to the thermal loads dissipated by the dissipative element.
In order to control the power level of the output power bus, a conventional shunt-type regulator apparatus sequentially steps through the solar array by shunting different ones of the solar array strings, either individually or in different combinations, until the power delivered to the output power bus is at the desired power level. Since a conventional shunt-type regulator apparatus must step through the solar array by sequentially connecting different combinations of the solar array strings to the output power bus, a shunt-type regulator apparatus is relatively slow to respond to changes in the power level of the output power bus and is limited by its slew rate.
As a result of the architecture of a conventional shunt-type regulator apparatus, the shunt-type regulator associated with one solar array string is not typically independent of the shunt-type regulators associated with the other solar array strings. As such, the failure of a single component can lead to the failure of the entire shunt-type regulator apparatus. Since a conventional shunt-type regulator apparatus includes a relatively large number of analog components, a conventional shunt-type regulator apparatus therefore generally has an undesirably large overall failure rate. Since solar arrays are typically disposed either in space or in terrestrial applications that are relatively remote, the failure of a conventional shunt-type regulator apparatus is particularly undesirable due to the difficulty in repairing or replacing the shunt-type regulator apparatus.
While a number of regulators have been developed for controlling the delivery of power by solar array strings to an output power bus, these conventional regulators suffer from a number of disadvantages. As such, it would be desirable to provide a regulator apparatus that is capable of quickly and controllably altering the power level of the output power bus by connecting any combination of the solar array strings to the output power bus. In addition, it would be desirable for a regulator apparatus to be resistant or tolerant to the failure of the regulators associated with one or more of the solar array strings such that the regulator apparatus can continue to operate despite these failures. Furthermore, it would be desirable for a regulator apparatus to include fewer components so as to be lighter, smaller and less expensive and to require generally less time for fabrication than a conventional regulator apparatus.